Cyclic voltammetry (CV) and scanning tunneling microscopy (STM) were used to examine the adsorption of o-, m-, and p-ethylaniline on single crystal Au(111) electrode in sulfuric acid. Each molecule resulted in three pairs of peaks in the CV profiles obtained in 0.5 M sulfuric acid containing 30 mM o-, m-, and p-ethylaniline, respectively. The features found at the most negative potentials were associated with the adsorption-desorption of these molecules. These peaks observed with o- and p-ethylaniline appeared at potentials more negative than that of m-ethylaniline, suggesting that the former were more strongly bonded to the Au(111) electrode, mostly likely via the amino groups.This is consistent with the fact that ethyl substituents on the benzene are electron-donating groups, which increase the electronic density at the amine group of these molecules.
Molecular resolution STM images were acquired to reveal the adlayer structures of these molecules on the Au(111) electrode. Generally, more and more ethylaniline molecules became adsorbed with more positive potential. Those structures observed at onset potentials of oxidation (~0.95 V) were most relevant to the polymerization of these molecules. In situ STM imaging conducted under potential control revealed fast oxidation and polymerization of o- and m-ethylaniline, but not p-ethylaniline. These results supported that these molecules could follow the head-to-tail coupling mechanism established for electrolymerization of anilines. The structures of o- and m-ethylaniline observed at 0.85 V (onset of polymerization) were (4√3 × 4√3) (4 × 2√3),respectively. Raising the potential to >0,95 V, these molecules were oxidized and polymerized with rather different molecular conformations, as revealed by the in situ STM. While o-ethylaniline produced ill-defined polymers, 3-ethylaniline resulted in linear chains of polymers preferentially aligned in the <112> directions of the Au(111) electrode surface. According to XPS and NEXAFS results, the nitrogen/sulfur elemental ratio in the poly3-ethylaniline film is 2/1, and the ratio of protonated /unprotonated nitrogen was 1/1, which is consistent with the expected value for the emeraldine salt of poly3-ethylaniline. The tilt angle defined by the benzene ring and the gold surface is approximately 47?.
I also explore the possibility of forming two-component polymers, such as polyethylaniline-aniline. In situ STM imaging at a fixed potential of 0.6 V showed that aniline and o-ethylaniline could be coadsorbed on Au(111) in 0.5 M sulfuric acid containing 30 mM of aniline and 30 mM o-ethylaniline. The ordered structures were identified as (3√3 × √43), (3√3 × √57), and (3√7 × √21), which were transformed into (3 × 2√3) as the potential was increased to 0.8 V. This structure was previously observed with aniline, suggesting that aniline, being more strongly adsorbed on Au(111) than o-ethylaniline, could displace foreigner admolecules and prevail. The conformation of the produced polymers at E > 0.9 V was the same as that of aniline, where linear chains were produced in the main axis of the Au(111) substrate. In the mixed system of aniline and 3-ethylaniline, in situ STM revealed a (4 × 2√3) structure at 0.8 V. Polymerization at E > 0.9 V resembles that of 3-ethylaniline, where straight chains were found in the <112> directions.